Backwards Inclined Fans
So had to change a drive out on a AHU with 2 fans. Got all done, programmed up, and checked operation. Both return and exhaust fans are over amping. Both direct drive and original. Open return and looped duct sock on supply. Have no blower specs, so I just measure velocity from openings. Stop the unit and look at the fans. They look like backwards incline fans, because of the fatter section of the blades normally goes into the air. So I reverse the command to the drives. Velocity jumps way up and amp draw goes down.
So now I'm thinking....why does amp draw go up when these were running backwards. Less air....less work....less amperage, no?
I never really ran one backwards and checked flow and amperage.
I take both direct drive and and original meens running on VFD or bypass. I have seen buildings with divercity built into the A/H system , meening it is not expected for all vav boxes to be open at the same time. If you loose the building you cant make static because all the boxes are open. I have seen guys put bigger pulleys on to try and troulble shoot this problem. the drive will limit on amperage and you get away with it until you go to bypass and then it over amps. Put the fan back into the corect rotation and limit your HZ on your drive to keep it on line long enough to trouble shut the static pressure and cfm. bottom line if it overamps in by pass the the pulleys are the wrong size.
There are two direct drive blowers(fans). They each are controlled by a VFD. There is no vav's or dampers on the system. The VFD's are just there to dial air flow in(since they are direct drive). I disabled current limiting on the vfd's while working on it. The fans run in one direction, move less air and over amp motors. The fans run in the other direction, move more air and less amperage.
Originally Posted by synergy
It just doesn't make sense to me.
Your right physics just won't allow for what you are saying. If your reading feet per minute with a manometor take an average reading in several places, maybe for some reason it draws the air differently going one way or the other. Either way I would back up my findings with a static pressure reading. Which ever direction makes the most static pressure is the correct rotation.
I have seen this as well on back. inclined fans ( new start -up ). I was lucky to find a rotation arrow and left it at that.
I'm guessing that because there is no " squirrel cage assembly " that a backward inclined axial wheel creates more turbulance and friction ( as seen by the motor ) when turning the wrong way ? I'm just guessing on this.
If you look at the wheel, the leading edge of the vane is closer to the center, trailing edge will be outside. Never ran one backwards past checking for rotation, so I don't know what they will do. My first thought was the same as the vav posted by Synergy.
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It sounds like what you are describing is an airfoil fan, rather than a backward inclined.
How they behave depends on if they are single width, single inlet open plenum fans like you would see in a FanWall array, or if they are double width, double inlet airfoil fans enclosed in a housing like you may see in a large VAV air handler lke a Trane Climate Changer. Enclosed airfoil fans are high velocity, high static fans. Plenum fans are also high static, but typically operate in the medium velocity range.
Both are what's referred to as non-overloading fans, I believe that means that they reach a point to the right of the curve where the bhp will not increase and they will not deliver any more air unless the static is decreased.
Speed and airflow are directly proportional to one another.
Static pressure varies with the square of any change in airflow.
Brake horsepower varies with the cube of any change in airflow.
With all centrifugal fans, when static goes up, volume goes down. This is just physics. If you were seeing the opposite, then it's either an instrumentation issue or had something to do with how you were taking the readings.
However, all bets are off if you're running it backwards. I've never tried it either, so I can't say for sure.
If we were talking in person, instead of explaining all this crap, I would have just shrugged my shoulders and said "I dunno" lol
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Backwards incline and airfoil fans work like a centrifugal pump. If they run backwards up go the amps and down goes the flow. It's like they dig into the air or water.